You must sacrifice exactly one creature to cast this spell; you can’t cast it without sacrificing a creature, and you can’t sacrifice additional creatures.

3/14/2017

Once you begin to cast Bone Splinters, no player may take actions until you’re done. Notably, opponents can’t try to remove the creature you wish to sacrifice.

3/14/2017

You can cast Bone Splinters targeting the creature you intend to sacrifice for its cost. This is because targets are chosen before costs are paid. Bone Splinters won’t resolve as it tries to resolve in this case.